Since glass has an excellent thermal stability and transparency as well as low vapor permeability, the glass have been used in the prior art as a substrate for the electronic display device such as a liquid crystal display device, organic electroluminescence (EL) display device, plasma display device and electronic paper, as a substrate for an electron-optical device such as a CCD or CMOS sensor, or a substrate for a solar battery. However, with the recent spread of a cellular phone and portable information terminal, there is an active demand for a substrate characterized by flexibility, light weight and resistance to cracks, to take place of the glass as a substrate which is comparatively heavy and susceptible to cracks.
In the liquid crystal display, birefringence is used for on-off operation of the display, and therefore, reduced birefringence is essential. Further, when various forms of functional thin film are formed on a plastic substrate, it is exposed to high temperature. This requires a high degree of resistance to heat. Especially when the transparent conductive film that is an essential functional thin film as a display substrate is to be formed into a low-resistant crystalline thin film, it is required to have a resistance to a high temperature of 180 degrees Celsius. Since all of these three physical properties are indispensable, it can be easily seen that many of the plastic substrates are not suited for display.
The material for the plastic substrate meeting such requirements is exemplified by polyether sulfone and polycarbonate. Especially, the Official Gazette of Japanese Patent Tokkaihei 5-142525 discloses a transparent film formed by lamination of a polyether sulfone and acryl based substrates, and recommends it to be used as a plastic substrate. As a result, this transparent film has come to be used in some cases. However, when the plastic substrate is used to manufacture a liquid crystal display and an organic electroluminescence display, it has been found out that such a product is accompanied by various faults that have not been observed in the glass substrate.
In the first place, unlike the glass substrate, the plastic substrate has the disadvantage of more or less allowing the passage of a gas such as vapor and oxygen that have an adverse effect on the devices constituting the display.
To overcome such disadvantages, an inorganic thin film having excellent gas barrier characteristics is used to cover the plastic substrate according to the vacuum deposition and sputtering method. It has been clarified that moisture permeability of the plastic substrate can be reduced by silicon oxide, for example.
However, the plastic substrate has a coefficient of thermal expansion a single to double digit greater than the glass substrate. This has resulted in a crack of a functional thin film arranged on the plastic substrate, or displacement of the pixel. It has been made clear that such a problem arises. In the aforementioned gas barrier type inorganic thin film, moisture permeability is drastically increased if a crack occurs. In the transparent conductive film, conductivity is reduced.
It is commonly known that the expansion coefficient of plastic film relative to heat and humidity can be reduced by drawing. Since drawing also increases the birefringence, a plastic substrate suited for use in the display cannot have been obtained even by drawing the aforementioned PES.
Thus, there has been no success in finding out a transparent plastic substrate having a low coefficient of thermal expansion in addition to three physical properties including low birefringence, high heat resistance and high transparency.
Further, a method of providing a gas barrier layer to control the passage of moisture or oxygen normally includes vacuum deposition, sputtering and vacuum plasma CVD methods. These methods involve use of complicated and bulky equipment, and have been characterized by low productivity and high cost in the arrangement of an inorganic thin film.
The first object of the present invention is to provide a transparent plastic film for use in a liquid crystal display, organic electroluminescence display or touch panel, the transparent plastic film being characterized by a high degree of transparency and heat resistance, as well as reduced birefringence and coefficient of thermal expansion.
The second object of the present invention is to provide measures for providing a gas barrier film of low moisture permeability and high durability by means of a simple process.
The third object of the present invention is to provide a liquid crystal display and a touch panel ensuring the minimum image distortion or color misregistration, and an organic electroluminescence display characterized by a high luminance of light emission.